Geometric profiles of aerofoils, including wings, propeller blades, rotor/turbine blades, stabilizers, and so on, are in general optimized for a design set of conditions or a design point or aspect of a mission profile, for example high lift, endurance, low drag and so on. Nevertheless, devices such as leading edge slats and/or trailing edge flaps, permit the geometry of the aerofoil to be varied to a limited degree while airborne, and thus enable operation of the aerofoil at other conditions.
However, aircraft that need at times to operate under a wide range of different conditions may incorporate aerofoil geometries that provide adequate performance for the range of conditions, albeit at a performance loss as compared with the optimum performance that may be obtained at any specific set of conditions with an aerofoil geometry designed for such a set of conditions.
Wing design for UAV applications requires a special approach which can be different from conventional manned aircraft. The main goal of operational UAV is often endurance flight at relatively low speed, and maximum lift is a primary factor to achieve high endurance values. Conventionally, an engineering compromise is reached between providing maximum lift, reducing drag penalty for high lift flight and providing flight envelope capabilities from low-lift maximum speed flight up to stall lift levels. Speed safety margins and acceptable stall characteristics are also factors conventionally considered in wing design.
In small UAV applications and at lower flight speeds, the aerodynamic flow over corresponding wings is typically low in air flow energy. This behavior, which is described by low values of Reynolds number, brings specific wing design features. The laminar-turbulent flow transition can become the critically important design point, different from conventional large-scale airplanes where the flow is usually turbulent. The two main parameters that define drag level for low Reynolds airfoil are: level of laminarity (extent of laminar flow before turbulent transition) and the size of laminar separation bubble. Increasing the laminar separation bubble may cause lift increment but on the other hand increases the possibility of bubble burst. The drag increases proportionally to separation bubble size, and the rear part of airfoil must provide enough length for pressure recovery after the laminar-turbulent transition, hence limiting airfoil laminar extent. For two-element airfoils there is the additional issue of very low local Reynolds numbers on the second element increasing risk of flow separation, especially when the second element is in a deflected position. The conventional engineering optimum is finding a compromise between the aforementioned parameters.
WO 2010/032241, assigned to the present Assignee, discloses aerofoil accessories configured for selective attachment to a wing element, the wing element having an outer facing aerofoil surface and being based on at least one datum aerofoil section. Each accessory provides a modified geometric profile to a datum profile of the at least one aerofoil section when attached to the wing element. The accessories are each configured for having a substantially fixed geometric profile with respect to the at least one datum aerofoil section at least whenever said wing element is airborne with the respective accessory attached to the wing element. The modified geometric profile is such as to provide said wing element with the accessory attached thereto with a desired change in performance relative to a datum performance provided by the wing element absent the accessory.
By way of general background, a number of inflatable or shape changing devices are known for altering the shape of aerofoils. For example, in U.S. Pat. No. 6,443,394, an airfoil device is provided for attachment to the wing of an aircraft. The airfoil device has a chamber which is inflatable to provide a lift-enhancing airfoil geometry to the wing and other chambers which are inflatable to provide deicing forces to remove ice accumulation on the wing. When installed on the wing, the airfoil device closely conforms to the wing's airfoil geometry (e.g., low camber, sharp leading edge) when the lift-enhancing chamber and the deicing chambers are in a deflated condition. The lift-enhancing chamber can be inflated during take-off and landing to provide a high camber and less sharp airfoil geometry. If ice accumulates on the wing during high speed flight, the deicing chambers can be repeatedly inflated/deflated for ice removal purposes. As another example, in U.S. Pat. No. 7,195,210, an airfoil member is provided including a geometric morphing device. The geometric morphing device has an inflatable member. The inflatable member has an exterior wall and multiple inflated states. Multiple layers are coupled to at least a portion of the exterior wall and control size, shape, and expansion ability of the geometric morphing device. The geometric morphing device is adjustable in size and shape by changing inflated state of the inflatable member. An airfoil member altering system and a method of performing the same are also provided as well as a method of forming the geometric morphing device.
There are also improvements that may be desired to be incorporated to an existing aerofoil design. For example, US2007/0278354, assigned to the present Assignee, and the contents of which are incorporated herein in their entirety, discloses a high lift, two-element, mild stall wing based on a corresponding high lift, two-element, mild stall aerofoils. An aerodynamic feature referred to as a mild stall ramp, or MS-ramp, is provided at the aft portion of the main body of the aerofoil, on the suction surface thereof, while retaining the bluntness of the leading edge of the aerofoil. Gradual development of separated flow on the MS-ramp, combined with continuous lift build-up at the forward portion of the aerofoil produces mild stall characteristics at the extended range of post-stall angles of attack, and combines features of adaptive geometry with stall/post-stall flight capabilities at the level of maximum lift that is inherent to two-element aerofoils.
The following publications are also provided by way of general background: U.S. Pat. Nos. 2,504,684; 2,937,826; 5,433,404; 6,786,457; and 6,910,661.